Coryneform bacteria were established as the classical amino acid producer after the discovery during the late 1950s that some strains of Corynebacterium glutamicum naturally excreted large amount of L-glutamic acid. This lead to an extensive screening and breeding programme to develop strains for the production of other amino acids. The genetic ＇tool box＇ for coryneform bacteria has been greatly expanded in recent years and almost all modern molecular biology techniques are now available for this organism. In coryneform bacteria, very few of the enzymes involved in amino acid production are controlled and no isoenzymes have been detected. Detailed information on the biosynthetic pathways and their regulation, and the availability of the genes involved now permit directed metabolic design, i.e. improvement of amino acid biosynthesis and excretion by manipulation of the enzymic and regulatory functions of coryneform bacterium with the application of recombinant DNA technology. The development of molecular cloning systems in these bacteria will enable the study of their genetic organization and also provide a powerful new tool for future strain improvements.